1. Field of the Invention
This invention relates to a catalyst for purifying an exhaust gas from an internal combustion engine in automobiles and the like, and to a process for producing it. More specifically, this invention pertains to an exhaust gas purification catalyst which removes carbon monoxide, hydrocarbons and nitrogen oxides from the above exhaust gas, suppresses the evolution of hydrogen sulfide, and at the same time, does not form a toxic substance, and to a process for its production.
2. Description of the Prior Art
Catalysts comprising active alumina having deposited thereon a platinum-group element such as platinum or rhodium and cerium oxide having an oxygen storage effect for increased low-temperature activity are now mainly used as a three-way catalyst for simultaneously removing hydrocarbons, carbon monoxide and nitrogen oxides from exhaust gases generated from internal combustion engines of automobiles and the like (Japanese Laid-Open Patent Publication No. 55225/1979). In such a conventional exhaust gas purification catalyst, sulfur dioxide (S.sub.02) formed by the combustion of sulfur-containing compounds in gasoline is usually stored on cerium oxide during running (stoichiometric A/F) and hydrogen sulfide is generated during idling (rich A/F). The hydrogen sulfide thus generated is released as an exhaust gas, which exhaust gas produces an offensive odor due to said hydrogen sulfide.
In order to suppress the evolution of hydrogen sulfide, it has mainly been the previous practice to add a nickel compound to the catalyst (Japanese Laid-Open Patent Publications Nos. 240949/1987 and 310637/1987). However, nickel compounds are not used in some European countries (particularly West Germany) on the ground that during use, they may possibly form toxic nickel carbonyl.
It has been desired therefore to develop an exhaust gas purification catalyst which efficiently removes carbon monoxide, hydrocarbons and nitrogen oxides from an exhaust gas, suppress the evolution of hydrogen sulfide, and at the same time, does not form a toxic substance.